This is the thesis of Robert Neuwirth, who studied from (and lived in) what he calls “the 21st Century Medieval City”. The video here below shows the results of his studies:

(The slides behind his speech can be found in a better resolution here, the original video is here)

According to him, these places are fully-developped cities, full of economical activities and with a surprisingly high level of organization. The thing that differenciate most them from the “official” city is the complete lack of debt: people build houses and infrastructure according to their financial possibilities, while most of the official city uses debt in order to grow.

And so, public powers should focus on the best way to integrate them in the official city. Neuwirth proposes these strategies:

Guarantee the “21st century medieval city” dwellers a right to stay. This will let them make long-term planning and allow them to build them better buildings.

Give political rights to the inhabitants of the “21st century medieval city”. This will allow them to lobby for their community and provide all the things the community needs.

Provide infrastructure. Infrastructures need a higher level of organization, that can be more easily provided by an organized political structure.

A result of these strategies can look like the images below:

(images: Naples Metro, Salvator Rosa station, from wikipedia)
A question remains open: how can all these strategies be integrated in a sustainable land use planning?(source: The Long Now Foundation)